Pneumatic and hydraulic cylinders are extensively used in actuator assemblies for moving implements, arms, booms and other components of mobile hydraulic machines such as tractors, combines, excavators, dozers, loader-backhoes, etc. For example, tractor-mounted implements such as plows are typically supported by hitch assemblies which include hydraulic cylinders for raising and lowering the implements. Harvesting heads on combines, blades on dozers, and buckets on loader-backhoes are further examples of implements typically positioned by hydraulic cylinders.
Electrohydraulic control systems for such actuator assemblies require position feedback signals representing the positions of the implements or mechanical joints being controlled. Some sensing assemblies (e.g., LVDTs) which provide position feedback signals are coupled to implements or mechanical joints using external linkages. External sensing assemblies, however, are subject to external impacts and other environmental influences.
The positions of implements or mechanical joints can also be determined using sensing assemblies internal to the cylinder. The internal sensing assemblies measure the extension of the cylinders which move the implements or mechanical joints. Cylinder extension is determined by measuring the position of the piston within the cylinder housing. The piston, in turn, moves a cylinder rod coupled to the implement or mechanical joint. The position of the implement or the joint is then determined as a function of piston position which depends upon the geometry of the particular mechanical system.
Various forms of apparatus for measuring positions of pistons within hydraulic cylinders are available. For example, the positions of pistons within cylinders have been measured using acoustic signals, radiofrequency (RF) signals and microwave signals with different sensing assemblies and circuit configurations. However, these apparatus suffer from such drawbacks as relatively high complexity and cost, relatively low reliability, durability and accuracy, and the need to extensively modify the cylinders to accommodate the sensing assembly.